Towards Sustainable, Environment-Friendly and Low-Waste Gas and Photoactivated Sensors for Low-Power Health and Air Monitoring and Medical Diagnostics

In the recent years, organic phototransistors (OPTs) have attracted considerable interest due to their unique capability of delivering flexible and wearable photosensor modules for various applications in the rapidly growing field of flexible electronics. Compared to conventional photodetectors and inorganic phototransistors, OPTs have an advantage of low-cost fabrication when it comes to their low processing temperature, as well as their mechanical flexibility and versatile chemical functionality of the organic materials. OPT structure is identical to that of an organic field-effect transistor (OFET), where the channel layer simultaneously plays the sensing role. The three-electrode geometry of an OPT controls and amplifies signal, while reducing noise and hence improves photosensitivity by simply adjusting the gate voltage (V_G).<br/>Recent advances on OPTs have been particularly focused on detection in the ultraviolet (UV) and near infrared (NIR) regions. UV sensors are gaining increasing attention for utilisation in the health NIR light-sensing OPTs has become one of the most important foundations for advanced control and sensing systems such as night vision for cars and airplanes, light detection and ranging (LiDAR) sensors for autonomous cars and drones, probe beams for biomedical devices and diagnostics, and optical communications and medical fields to prevent sunburn and skin cancer caused by sunlight. Numerous researches have been dedicated to NIR sensors for medical diagnostics, for instance, pulse oximetry or monitoring blood pressure. Home-assisted (bio)medical sensors have gained more demand than ever before, due to increased hospitalisation and hospitals overcrowding during the Covid-19 pandemic. Photoplethysmography (PPG) sensors can be integrated into home-assisted kits and offer tremendous advantage to monitor cardiovascular and atherosclerosis disease markers, such as heart rate (HR), blood pressure (BP), heart rate variability (HRV), arterial oxygen saturation (SpO₂), arterial ageing. Most basic human characteristics can be evaluated by the flow of blood in the subcutaneous tissue. PPG sensors can detect these indicators by monitoring pulsatile changes in blood flow. However, the NIR-absorbing organic materials are very rare because of the difficulty in synthesis to meet the narrow energy band (level) gap of ca. 0.89~1.65 eV, which corresponds to the wavelength (λ) range of ca. 750~1400 nm. In this regard, conjugated donor-acceptor (DA) polymers have been considered viable NIR-absorbing materials since their energy band gaps can be altered by combinations of electron-donating and electron-accepting monomers. Moreover, when used as gas and bio-sensors, sensitivity, selectivity and drift control are critical parameters of consideration.<br/>In this work, we proposed novel, low-power, highly sensitive, sustainable OFETs and OPTs based on naturally occurring, biodegradable and low-waste materials. We applied and modified aloe vera (aloin) and cellulose as natural dielectric materials to operate devices at low voltages. For the active material, we synthesised a selection of DA polymers based on thieno-thiophene (TT) and naphthalene diimide (NDI) as the donor and acceptor respectively, with different substituents, as well as metal phthalocyanine, to verify their electrical and sensing performances. The dielectric layers are separately characterised in parallel-plate capacitors and electrical and sensing (both as light-receiving device and chemical sensors) of the OFETs are fully tested. We believe that these natural, low-waste OFET devices can be used as gas, chemical and photoactivated light-receiving, sensing devices with low-operating voltage and good environmental and bias stress stability.